Mobile wireless communications systems have developed from first generation analogue based mobile wireless systems to second generation digital systems such as the Global System for Mobile Communications (GSM). Now, the so-called third generation mobile communications system is being evolved. This is a next generation global mobile communications system that is generally based on the core network technology of GSM but employing a distinct wireless access technology. Various telecommunications standards bodies and telecommunications equipment vendors from around the world have agreed to collaborate on what is known as the Third Generation Partnership Project (3GPP). The 3GPP encompasses a number of research fields relating to development of the Universal Mobile Telecommunications System (UMTS). UMTS is the European form of the third generation mobile communications system.
GSM was developed as a voice based mobile wireless communications technology. Some years ago it was enhanced by the addition of a data technology known as General Packet Radio Service (GPRS) which enables GSM mobile devices (handsets) to send and receive packet based messages.
UMTS is a new wireless access technology that is quite different and thus generally incompatible with GSM/GPRS but which affords considerably higher data rates than GPRS. However, because of the high capital costs of installing a replacement UMTS system, such systems are gradually being deployed in co-existence with the already installed GSM systems. As such, for some time to come, mobile wireless communications systems will consist of both GSM/GPRS and UMTS services with many areas supporting only GSM/GPRS. Consequently, it is necessary that mobile devices support both technologies and be able to switch between these in either direction. Only one of the technologies is active at any time for a connection although handover between technologies for a currently active connection is enabled.
The 3GPP specifications for GPRS/UMTS describe how data packets should be transferred on each technology and when a switch between technologies should occur for a packet data (i.e. packet switched) connection. In most modes of operation, the specifications describe data being transferred through the Internet Protocol (IP) layer of the protocol stack of the active technology path with no simple process for determining whether data transmission through the stack is successful. If a data packet does not get transmitted by a lower layer of the active protocol stack, which occurs frequently due to poor radio conditions, for example, and, if a technology switch is then effected, such data packet may be lost in the initially active data path. This is particularly egregious for delay sensitive packet data connections such as conversational class real time services, e.g. video telephony.
The 3GPP specifications do describe for GPRS and UMTS a number of modes of operation comprising one or more of a “transparent mode”, an “unacknowledged mode” and an “acknowledged mode”. The acknowledged mode of operation does involve a packet transmission acknowledgement process. However, the described process involves latency issues for delay sensitive services and thus is a rarely employed mode of operation.